1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to measurement of voltage levels of voltage sources involving conversion of voltage values.
2. Description of the Related Art
In voltage measurement applications, a voltage which is monitored may include an offset voltage from an offset voltage source, such as where a plurality of electrochemical cells are connected in series. By connecting a plurality of electrochemical cells in this manner, the voltage of each of the individual cells is added. Accordingly, the total voltage of the series connected electrochemical cells may be provided as a power source for applications where the desired voltage exceeds the voltage of an individual cell.
Further, many applications require operating voltages that are significantly higher than the voltage of a single cells. In these applications, numerous individual electrochemical cells may be connected in series to provide the required operating voltage. Also, many applications which utilize electrochemical cells involve using cells that are rechargeable, such as lithium ion cells. In these applications, the voltage of the cells is monitored to determine when the cells require recharging. Further, the voltages of the individual cells are monitored to regulate the charging such that overcharging of the cells, which could damage the cells, does not occur.
In addition, when a plurality of cells are connected in series to provide a power source, it is desirable to monitor the voltage level of each of the cells individually. Because the total voltage capacity and the discharge rate of each cell may vary with respect to other cells in the series connection, the voltage of some cells may be lower than others. Accordingly, the efficiency of a circuit may be diminished due to less than optimal charging of each of individual cells.
Therefore, monitoring circuits have been developed which allow each cell voltage to be monitored. For example, prior approaches involve the use of resistor divider circuits and differential amplifier circuits which measure the voltage of the individual cells. Given a measurement of each individual cell, the voltage value may be provided to a charge controlling circuit which accepts inputs from the measurement circuits for each of the electrochemical cells. For example, a charge controlling circuit may be a microcontroller or other processor which regulates a charging current that is applied to each of the cells when it is determined that a cell requires charging. Further, the amplitude of the measure voltage may be provided as an analog input which is then converted to a digital value for processing by the charge controlling device.
Typically, monitoring circuits accept input voltage levels that are substantially lower than the total voltage that is observed from an individual cell voltage with the offset voltage produced by other cells in the series connection. For instance, input voltage levels may typically be limited to a maximum of 5 volts. Accordingly, it is frequently necessary to convert the total voltage of each cell in a series connection down to a voltage value that represents the voltage of the cell without the presence of the offset voltage from the other cells.
However, monitoring circuits based upon differential amplifiers and resistor divider circuits suffer from excessive current draw as the number of cells are added to the series string increases, thereby resulting in higher background energy losses. In many applications, such excessive current draw leads to substantial reduction in the operating time of battery powered circuits. Thus, the efficiency of the device may be decreased.
In addition, measuring circuits which utilize resistors connected in a plurality of resistor divider networks suffer from accuracy errors due to attenuation of the measurement signal which reduces resolution in measuring the signal. Therefore, resistor divider measuring circuits are unable to accurately determine small deviations in voltage levels of the individual cells. Further, commercially available amplifiers and multiplexors which are used in conventional monitoring circuits are limited by the supply voltage which must be provided from the cells in order to operate the devices.
Also, conventional approaches suffer from a lack of accuracy when the offset voltage varies. Thus, increased offset voltage levels resulting from a large number of series connected cells may give rise to an increasing error in the measured voltage. Measurement error also results in conventional approaches due to variations in operating temperature of the measurement circuit.
Additionally, different approaches have been put forward that involve putting a smaller collection of cells in modules that do not violate the limits of the integrated circuits. However, this approach raises the cost of the components in the measurement circuit because of the increased number of microprocessors for each of the smaller modules and reduces the ability to build standalone cell batteries which have a series connection of 16 cells or more.